In a central office switch, line cards are typically supplied with voltages that are derived from a single -48V battery source. For example, 5V DC power to the line card, converted from the -48V, supplies operating voltage for logic, and -48V DC filtered power, commonly referred to as "battery voltage", supplies the line power used for talking and signaling on/off-hook.
Loss of battery voltage is typically detected at a frame or shelf level when protection fuses blow. This is considered a major event causing various alarms to trip and power failure routines to kick in.
A loss of battery voltage caused by a catastrophic problem obviously should be treated as a major alarm. However, localized losses of battery voltage at a line card may also cause a substantial drain on system microprocessor resources. For example, a permanent loss of battery voltage at a line card may be interpreted by the system microprocessor as an off-hook condition for each line served by the line card, and the microprocessor could be flooded by false off-hook signals. Also, a localized loss of battery may be the consequence of a normal system activity, such as a hot swap of a nearby circuit board causing a temporary discharge of the local decoupling capacitors. This loss of battery also may be interpreted as an off-hook condition by the system microprocessor, causing the microprocessor to be flooded by false off-hook signals.